Aluminum alloy sheet having excellent ridging resistance and hem bendability and production method for same

ABSTRACT

The present disclosure relates to an aluminum alloy sheet having excellent ridging resistance and excellent hem bendability. The aluminum alloy sheet is an aluminum alloy rolled material for molding. The aluminum alloy rolled material includes an aluminum alloy that includes predetermined amounts of Mg, Si, and Mn and satisfies a condition of 0.005≤Mn+Cr≤0.080 mass %. In the aluminum alloy sheet, the position of a middle (t/2) of a sheet thickness is regarded as a center, the crystal particle diameter d 1  of an L-LT plane in a sheet thickness in a range of ±(t/8) from the center is 30 to 80 μm, the crystal particle diameter d 2  of an L-ST plane in the entire sheet thickness is 60 μm or less, and the cube orientation area rate C of a crystal orientation on a sheet surface is 10% or more.

TECHNICAL FIELD

The present disclosure relates to an aluminum alloy sheet that is preferably used in the members and components of various automobiles, ships, aircraft, and the like, such as automobile body sheets and body panels, as well as construction materials, structural materials, various machinery and appliances, household electrical appliances, the components thereof, and the like. In particular, the present disclosure relates to: an aluminum alloy sheet with excellent ridging resistance and excellent hem bendability, which is preferred for the applications; and a method for producing the aluminum alloy sheet.

BACKGROUND ART

Demands for improvement in fuel efficiency through a reduction in the weights of automobiles have been increased against recent requirements such as suppression of global warming and a reduction in energy costs as backgrounds. In response to the demands, aluminum alloy sheets have also increasingly tended to be used as automotive body sheets applied to automobile body panels, in place of conventional cold rolled steel sheets. An aluminum alloy sheet has a specific gravity about one-third the specific gravity of a conventional cold rolled steel sheet while having a strength approximately equivalent to the strength of the conventional cold rolled steel sheet, and can contribute to a reduction in the weight of an automobile. Aluminum alloy sheets have also been recently often used in molded components such as the panels and chassis of electronic and electrical instruments and the like, in addition to automotive applications. Like automotive body sheets, such aluminum alloy sheets have been often pressed and used.

Commonly, Al—Mg—Si-based alloys and Al—Mg—Si—Cu-based alloys as well as Al—Mg-based alloys have been primarily used as aluminum alloy sheet materials for automobile body sheets. The Al—Mg—Si-based alloys and the Al—Mg—Si—Cu-based alloys, which are alloys having aging properties, result in improvement in strength after coating baking, in comparison with a strength before the coating baking, by using a heating step in the coating baking. In other words, the Al—Mg—Si-based alloys and the Al—Mg—Si—Cu-based alloys have been recently increasingly applied to automotive materials because of having an advantage that a relatively low strength and excellent formability are achieved before the coating baking while a higher strength is achieved after the coating baking.

The aluminum alloy sheet materials for automobile body sheets have particularly required excellent formability and excellent surface quality. With regard to the formability of the aluminum alloy sheet materials, the edges of sheets have been commonly often subjected to hemming-bending in the case of integrating outer and inner panels. The hemming-bending can be considered to be very severe working for a material because 180-degree bending is performed at an extremely small bending radius. Thus, it has been strongly required that the aluminum alloy sheet materials for automobile body sheets have particularly had hem bendability among types of formability.

The surface quality of an aluminum alloy sheet material is a feature characterizing appearance quality after molding. With regard to the surface quality, Al—Mg—Si-based alloys and Al—Mg—Si—Cu-based alloys have an advantage that generation of a Lueders mark which has been problematic in Al—Mg-based alloys is inhibited. However, the aluminum alloys also often have a problem that a ridging mark in which recesses and projections having a stripe shape are formed on a sheet surface after press molding is generated.

The ridging mark is a fine recessed and projected pattern that appears in a stripe shape in a direction parallel to the direction of rolling in a step of producing a sheet as a material when the sheet is molded. The ridging mark is particularly prone to be generated under a severe press molding condition. A material on which a ridging mark is prevented from being generated has been strongly demanded with increasingly demanding the complicated and thinned shapes of automobile bodies in recent years. Herein, resistance to generation of a ridging mark in molding is referred to as “ridging resistance”.

The generation of the ridging mark is profoundly associated with a recrystallization behavior in a material. Therefore, control of a metallographic structure in a process of producing a sheet has been considered to be essential for suppressing the generation of the ridging mark. Thus, for example, proposals from the viewpoint of controlling a recrystallization state in a step of hot-rolling a sheet material, the viewpoint of controlling a crystal orientation, and the viewpoint of controlling the crystal particle diameter of a product sheet, as described in Patent Literature 1 to 5, have been made as conventional technologies for improving ridging resistance.

For example, proposals from the viewpoint of a crystal orientation, as described in Patent Literature 6 and 7, have been made with regard to improvement in hem bendability.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 2823797

Patent Literature 2: Japanese Patent No. 3590685

Patent Literature 3: Unexamined Japanese Patent Application Kokai Publication No. 2009-263781

Patent Literature 4: Unexamined Japanese Patent Application Kokai Publication No. 2010-242215

Patent Literature 5: Unexamined Japanese Patent Application Kokai Publication No. 2014-234542

Patent Literature 6: Japanese Patent No. 5113318

Patent Literature 7: Japanese Patent No. 4939091

SUMMARY OF INVENTION Technical Problem

Further improvement of material quality, particularly surface appearance quality, has been recently demanded in view of design quality and the like. Especially, it is strongly required that an Al—Mg—Si-based alloy sheet and an Al—Mg—Si—Cu-based alloy sheet having a strength improvement characteristic (bake hardenability) in a coating baking step have superior ridging resistance. As a matter of course, it is required that the Al—Mg—Si-based alloy sheet and the Al—Mg—Si—Cu-based alloy sheet have formability such as hem bendability described above, as well as ridging resistance. However, it has been difficult for the conventional technologies described above to sufficiently satisfy such required performances.

In methods for producing an aluminum alloy sheet, described in Patent Literature 1 and 2, a temperature at which hot rolling is started is set in a range of 350 to 450° C., and therefore, the formation of coarse crystal grains during the hot rolling is suppressed to some extent. However, the effect of the suppression has been insufficient. An experiment conducted by the present inventors has revealed that in particular, coarse crystal grains are formed in the vicinity of the middle of a sheet thickness, and consequently, sufficient ridging resistance is not necessarily obtained.

In methods for producing an aluminum alloy sheet, described in Patent Literature 3 and 4, the particular crystal orientation of an aluminum alloy sheet is controlled to eliminate a structure in which the group of crystal grains of which the crystal orientations are similar, causing a ridging mark, is formed in a rolling direction. The methods have certain effects for improving ridging resistance. However, the effects are insufficient for recently increasing, further demands for improvement in ridging resistance.

Patent Literature 5 proposes that ridging resistance is improved by controlling a crystal particle diameter. However, balancing with bendability has been insufficiently examined in the aluminum alloy sheet produced by this method. In the method for producing an aluminum alloy sheet, differential speed rolling and plural intermediate annealing treatments as well as usual hot rolling and cold rolling have been required, and a very complicated production step has been needed.

Patent Literature 6 and 7 propose that hem bendability which is a feature important for an automobile body sheet material is greatly improved by growing a cube orientation. However, the improvement of ridging resistance and the improvement of hem bendability by control of a crystal orientation require structure controls contradictory to each other, and the achievement of the structure controls requires use of a very complicated and high-cost production step.

In the conventional Al—Mg—Si-based and Al—Mg—Si—Cu-based alloy sheets, no sufficient effect for improving ridging resistance and hem bendability has been exhibited, and, in addition, it has also been difficult to achieve both of these features, as described above. In addition to the solution of such problems, there have recently also been challenges to address demands for reducing the production costs of alloy sheet materials. The development of a technology to inexpensively produce an automobile body sheet material of which the performance is higher than those of conventional products has been strongly required. In this regard, examples of a manner for reducing the cost of an alloy sheet material include the omission of some of production steps. However, such an easy manner has not been adopted because of resulting in the deterioration of various performances such as ridging resistance, hem bendability, and bake hardenability required by automobile body sheet materials.

The present disclosure was made under such circumstances and is to provide an aluminum alloy sheet for molding that has excellent ridging resistance, can allow the generation of a ridging mark to be reliably suppressed even under a severe molding condition, and also has excellent hem bendability. An objective of the present disclosure is to also provide a production method by which such an aluminum alloy sheet for molding that has excellent performance can be reliably and stably produced at low cost on a mass production scale.

Solution to Problem

According to an examination conducted by the present inventors, examples of the causes of generating a ridging mark in an aluminum alloy sheet include a band-shaped structure (stripe-shaped structure) formed by crystal grains enlarged in a rolling direction in a hot rolling step and a cold-rolling step. Thus, the suppression of the formation of the band-shaped structure or the decomposition of the band-shaped structure before producing a production sheet is required for suppressing the generation of a ridging mark. Conceivable examples of steps in which the action of the decomposition of the band-shaped structure can be expected include a solution treatment step performed after the cold-rolling step. In the solution treatment step, recrystallization proceeds, and therefore, recrystallized grains generated by the recrystallization can decompose the band-shaped structure.

The present inventors repeatedly examined a method for effectively decomposing a band-shaped structure becoming the origin of a ridging mark. As a result, it was found that the power to decompose the band-shaped structure is increased by increasing the particle diameters of recrystallized grains generated by recrystallization. It can be considered that the band-shaped structure is decomposed by the coarse recrystallized grains, whereby strong linearity in a rolling direction, which is a feature of a ridging mark, is greatly decreased, and consequently, the generation of the ridging mark can be suppressed.

As a result of further repeating examinations, the present inventors found that a band-shaped structure formed in the case of hot rolling is allowed to be fine as a technique for enhancing the effect of decomposing a band-shaped structure by recrystallized grains. In addition, the present inventors also considered that a decrease in hot-rolling temperature is effective for allowing a band-shaped structure formed in a hot-rolling step to be fine.

According to the present inventors, a decrease in hot-rolling temperature also leads to improvement in bendability. In other words, a rolling texture can be grown by cold rolling at a sufficient rolling reduction after hot rolling at a low temperature. The grown rolling texture can contribute to the growth of a cube orientation in solution treatment. A decrease in hot-rolling temperature can be considered to be related to improvement in bendability because improvement in the cube orientation density of a product sheet leads to the possibility of improving bendability as described above. However, such texture control may commonly limit some of production steps and precludes the construction of a production method by which another feature is simultaneously improved.

In other words, the texture control is performed by adjusting working and a heat history, and the working and the heat history may restrict each other. In the present disclosure, it is required to coarsen recrystallized grains in solution treatment simultaneously with the texture control. Particle-diameter control for coarsening recrystallized grains may require a reduction in the strain energy accumulated by rolling due to heat treatment (for example, performance of intermediate annealing) other than solution treatment, and such heat treatment may cause the inhibition of the formation of a rolling texture contributing to the growth of a cube orientation. Accordingly, it is considered that it is difficult to achieve both improvement in bendability due to improvement in cube orientation density and improvement in ridging resistance due to an increase in recrystallization particle diameter in conventional aluminum alloys (Al—Mg—Si-based alloys and Al—Mg—Si—Cu-based alloys) although both the improvement in bendability and the improvement in ridging resistance can be achieved by decreasing a hot-rolling temperature.

Thus, the present inventors repeated the intensive examination of means for achieving both improvement in ridging resistance based on coarsening of recrystallized grains and improvement in bendability based on the growth of a cube orientation due to texture control. As a result, it was decided to also additionally perform an examination from the viewpoint of constituent compositions in an Al—Mg—Si-based alloy and an Al—Mg—Si—Cu-based alloy. Specifically, it was found that both the improvement in ridging resistance and the improvement in bendability can be achieved by restricting the amounts of Mn and Cr in view of the constituent elements of the aluminum alloys.

In the Al—Mg—Si-based alloy and the Al—Mg—Si—Cu-based alloy, Mn and Cr are elements that suppress the growth of crystal grains. The restriction of the amounts of these added Mn and Cr to lower levels than usual enables the promotion of an increase in recrystallization particle diameter in solution treatment and the decomposition of a band-shaped structure, thereby improving ridging resistance. In the Al—Mg—Si-based alloy and the Al—Mg—Si—Cu-based alloy in which the amounts of these added Mn and Cr are restricted, an increase in crystal particle diameter as well as the growth of a cube orientation in solution treatment is achieved by selecting a step of growing a rolling texture by hot rolling and cold rolling.

As a result of repeating various experiments and examinations, the present inventors found that ridging resistance and hem bendability are reliably and prominently improved by restricting the amounts of Mn and Cr in an Al—Mg—Si-based alloy and an Al—Mg—Si—Cu-based alloy to lower levels than usual and controlling the crystal particle diameters and crystal orientation of a final sheet by controlling a hot-rolling temperature, omitting intermediate annealing, adopting a sufficient cold rolling reduction, and performing solution treatment in a method for producing the Al—Mg—Si-based alloy and the Al—Mg—Si—Cu-based alloy, as described above.

In other words, the present disclosure provides an aluminum alloy sheet with excellent ridging resistance and excellent hem bendability, the aluminum alloy sheet including: an aluminum alloy including 0.20 to 1.50 mass % Mg, 0.30 to 2.00 mass % Si, and one or two of 0.005 to 0.080 mass % Mn and 0.005 to 0.080 mass % Cr, satisfying 0.005≤Mn+Cr≤0.080 mass %, and including the balance Al and inevitable impurities, wherein the aluminum alloy sheet has a sheet thickness t; the position of the middle (t/2) of the sheet thickness is regarded as a center, and the crystal particle diameter d1 of an L-LT plane in a sheet thickness in a range of ±(t/8) from the center is 30 to 80 μm; the crystal particle diameter d2 of an L-ST plane in the entire sheet thickness is 60 μm or less; and the cube orientation area rate C of a crystal orientation on a sheet surface is 10% or more.

The aluminum alloy included in the aluminum alloy sheet of the present disclosure further includes one or more of 0.01 to 0.40 mass % Zr, 0.03 to 1.00 mass % Fe, 0.005 to 0.300 mass % Ti, and 0.03 to 2.50 mass % Zn, and includes Cu that may be restricted to 1.50 mass % or less.

In the present disclosure, a method for producing the aluminum alloy sheet according to the present invention is a method for producing the aluminum alloy sheet with excellent ridging resistance and excellent hem bendability, the method including: a casting step of casting the aluminum alloy; a hot-rolling step of hot-rolling an ingot to form a hot-rolled sheet; a cold-rolling step of cold-rolling the hot-rolled sheet without subjecting the hot-rolled sheet to intermediate annealing, to form a cold-rolled sheet; and a solution treatment step of performing solution treatment of the cold-rolled sheet, wherein in the hot-rolling step, a hot-rolling start temperature is set at 300 to 450° C., and a hot-rolling end temperature is set at 200 to 450° C.; in the cold-rolling step, the cold-rolled sheet having a final sheet thickness is formed at a rolling reduction of 50.0% or more; and in the solution treatment step, the solution treatment of the cold-rolled sheet is performed at a temperature of 480 to 590° C.

In the production method described above, the hot-rolling start temperature may be set at 300 to 450° C., and the hot-rolling end temperature may be set at 200 to 350° C. In the production method described above, the hot-rolling start temperature may be set at 350 to 450° C., and the hot-rolling end temperature may be set at more than 350° C. and 450° C. or less.

The production method described above may further include a homogenization treatment step of performing homogenization treatment of the ingot at a temperature of 480 to 590° C. for 0.5 to 24 hours between the casting step and the hot-rolling step.

Advantageous Effects of Invention

According to the present disclosure, there can be provided an aluminum alloy sheet that can allow the generation of a ridging mark to be reliably suppressed even under a severe molding condition and that has excellent hem bendability. In a method for producing an aluminum alloy sheet according to the present disclosure, an aluminum alloy sheet having excellent ridging resistance and excellent hem bendability can be produced at a cost lower than that in a conventional production method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram of a measurement plane with particle diameters d1 and d2 and a cube orientation area rate C in the present invention.

DESCRIPTION OF EMBODIMENTS

An aluminum alloy sheet having excellent ridging resistance and excellent hem bendability according to the present disclosure, and a method for producing the aluminum alloy sheet will be described in detail below. In the following description, the constituent composition of an aluminum alloy included in the aluminum alloy sheet according to the present disclosure will be described, and the controls of the crystal particle diameters and crystal orientation of the aluminum alloy sheet will be described. In addition, the method for producing an aluminum alloy sheet, including a step of performing such controls of crystal particle diameters and a crystal orientation, will be described. Herein, a simple expression of “%” for describing the constituent composition of the alloy means “mass %”.

1. Constituent Composition of Aluminum Alloy

The aluminum alloy sheet according to the present disclosure includes an Al—Mg—Si-based alloy or an Al—Mg—Si—Cu-based alloy, and the constituent composition of the aluminum alloy includes 0.20 to 1.50% Mg, 0.30 to 2.00% Si, and one or two of 0.005 to 0.080% Mn and 0.005 to 0.080 mass % Cr, and satisfies 0.005 mass %≤Mn+Cr≤0.08 mass %. The aluminum alloy of the present disclosure, further including one or more selected from 0.01 to 0.40% Zr, 0.03 to 1.00% Fe, 0.005 to 0.300% Ti, and 0.03 to 2.50% Zn, and including Cu restricted to 1.50% or less, and the balance Al and inevitable impurities, is also preferably used. The reason why each of the elements described above is limited will now be described.

Mg:

Mg, which is a fundamental alloy element of the Al—Mg—Si-based or Al—Mg—Si—Cu-based alloy targeted in the present disclosure, contributes, together with Si, to improvement in strength. A Mg content is set at 0.20 to 1.50%. A Mg content of less than 0.20% results in a decrease in the amount of generated G.P. zone contributing to improvement in strength by precipitation hardening in coating baking, and therefore prevents the sufficient effect of improving strength from being obtained. In contrast, a Mg content of more than 1.50% results in the generation of a coarse Mg—Si-based intermetallic compound, thereby deteriorating press formability, primarily bending workability. A Mg content set at 0.30 to 0.90% is preferred for particularly allowing the bending workability of a final sheet to be more favorable.

Si:

Si, which is also a fundamental alloy element of the Al—Mg—Si-based or Al—Mg—Si—Cu-based alloy targeted in the present disclosure, contributes, together with Mg, to improvement in strength. Si is generated as a crystallized product of a metal Si particle in casting. A Si content is set at 0.30 to 2.00%. A Si content of less than 0.30% prevents the above-described effect from being sufficiently obtained. In contrast, a Si content of more than 2.00% results in the generation of coarse Si particles and a coarse Mg—Si-based intermetallic compound, thereby causing the deterioration of press formability, particularly bending workability. A Si content set at 0.50 to 1.30% is preferred for obtaining a more favorable balance between press formability and bending workability.

Mn and Cr:

Mn and Cr contribute to improvement in strength and to the refinement and stabilization of a crystal grain structure. In the present disclosure, however, the contents of these additional elements are strictly restricted from the viewpoint of controlling the crystal particle diameters of a final sheet. A Mn content is set at 0.005 to 0.080%, and a Cr content is set at 0.005 to 0.080% in order to obtain the crystal particle diameters defined in present disclosure. Mass production stability is poor when both Mn and Cr are less than 0.005%, while the crystal particle diameters become fine, and it is difficult to obtain the crystal particle diameters defined in the present disclosure when both Mn and Cr are more than 0.080%. In the present disclosure, one or two of Mn and Cr are included.

It is required that the contents of Mn and Cr further satisfy 0.005≤Mn+Cr≤0.080%. It is necessary to also restrict the total of the Mn and Cr amounts in order to obtain the crystal particle diameters defined in the present disclosure. A crystal particle diameter is excessively increased when Mn+Cr is less than 0.005%, while a crystal particle diameter is excessively decreased when Mn+Cr is more than 0.080%. It is preferable that Mn+Cr satisfies 0.010%≤Mn+Cr≤0.050%. A decrease in the contents of Mn and Cr also leads to improvement in hem bendability and formability because Mn and Cr allow an intermetallic compound resulting in the deterioration of hem bendability and formability to be formed in a material.

Zr, Fe, Zn, and Ti:

These elements are effective for improvement in strength, crystal grain refinement, improvement in aging property (bake hardenability), and/or improvement in surface treatability, and it is preferable that any one or more of the elements are included. Among these elements, Zr exhibits the effect of the improvement in strength and the refinement and stabilization of the crystal grain structure, described above. A Zr content of less than 0.01% prevents the above-described effect from being sufficiently obtained. In contrast, a Zr content of more than 0.40% results not only in the saturation of the above-described effect but also in the generation of a large number of intermetallic compounds, thereby causing the possibility of deteriorating formability, particularly hem bendability. Accordingly, the content of Zr is preferably set at 0.01 to 0.40%. The content of Zr is more preferably set at 0.01 to 0.30%.

Fe is also an element effective for improvement in strength and for crystal grain refinement. An Fe content of less than 0.03% prevents the above-described effect from being sufficiently obtained. In contrast, an Fe content of more than 1.00% results in the generation of a large number of intermetallic compounds, thereby deteriorating press formability and bending workability, and in an extreme decrease in crystal particle diameter after solution treatment, thereby precluding the obtainment of the crystal particle diameters defined in the present disclosure. Accordingly, the Fe content is preferably set at 0.03 to 1.00%. An Fe content set at 0.05 to 0.50% is more preferred for particularly minimizing the deterioration of the bending workability and facilitating the obtainment of the crystal particle diameters defined in the present disclosure.

In addition, Zn is an element that contributes to improvement in strength through improvement in aging property and that is effective for improving surface treatability. A Zn content of less than 0.03% prevents the above-described effect from being sufficiently obtained. In contrast, a Zn content of more than 2.50% results in the deterioration of formability. Accordingly, the Zn content is preferably set at 0.03 to 2.50%. The Zn content is more preferably set at 0.03 to 1.00%.

Ti exhibits the effect of the refinement of an ingot structure. A Ti content is preferably set at 0.005 to 0.300%. A Ti content of less than 0.005% prevents the above-described effect from being sufficiently obtained. In contrast, a Ti content of more than 0.300% results not only in the saturation of the effect of the addition of Ti but also in the possibility of generating a coarse crystallized product. The Ti content is more preferably set at 0.005 to 0.200%. The addition of 500 ppm or less of B, simultaneous with the addition of Ti, results in the still more noticeable effect of the refinement and stabilization of the ingot structure.

Cu:

Cu is an element that may be added for improving strength and formability. However, a Cu content of more than 1.50% results in the deterioration of corrosion resistance (intergranular corrosion resistance and filiform corrosion resistance). Accordingly, the Cu content is preferably restricted to 1.50% or less. The Cu content is preferably restricted to 1.00% or less when it is intended to attempt further improvement of corrosion resistance. The Cu content is preferably restricted to 0.05% or less when corrosion resistance is particularly regarded as important.

In addition to each of the above elements, B, Ca, Na, and the like of which the amount of each is less than 0.05% and the total is less than 0.15% may be included as inevitable impurities because of having no influence on the features of the aluminum alloy sheet according to the present disclosure.

The above-described contents of Zr, Fe, Zn, and Ti are described as ranges in the case of positively adding each of Zr, Fe, Zn, and Ti. Accordingly, a case in which these elements of which the amounts are less than the lower limit values of the above-described contents are included as impurities is not excluded. Particularly with regard to Fe, less than 0.03% Fe is typically included as an inevitable impurity when a usual aluminum base metal is used.

2. Controls of Crystal Particle Diameters and Crystal Orientation of Aluminum Alloy Sheet

In the aluminum alloy sheet according to the present disclosure, it is also very important to control the crystal particle diameters and crystal orientation of the aluminum alloy sheet which is a final sheet. The ridging resistance and hem bendability of the final sheet are reliably and stably improved by controlling the crystal particle diameters and crystal orientation of the alloy sheet while adjusting the constituent composition of the alloy. Particle diameters d1 and d2 and a cube orientation area rate C intended to define the aluminum alloy sheet according to the present disclosure will be described in detail below while referring to FIG. 1 illustrating positions at which the particle diameters d1 and d2 and the cube orientation area rate C are measured.

2-1. Technological Significance of Controls of Crystal Particle Diameters and Crystal Orientation

First, the technological significance of the control of the crystal grains of the aluminum alloy sheet will be described. According to the present inventors, a band-shaped structure particularly having a strong influence on the generation of a ridging mark is present in a region in the vicinity of the middle in the sheet thickness direction of the aluminum alloy sheet. Thus, such recrystallization that a crystal particle diameter in the region is increased to an appropriate size causes the decomposition of the band-shaped structure to be promoted, thereby preventing the generation of a ridging mark. Herein, the region in the vicinity of the middle in the sheet thickness direction refers to a region in a range of ±(t/8) from a center in the thickness direction assuming that the sheet thickness is t, and the position of the middle (t/2) of the sheet thickness is regarded as the center (hereinafter, the region may be referred to as “region in vicinity of middle”).

According to an experiment conducted by the present inventors, the setting of the crystal particle diameter d1 in an L-LT plane in the region in the vicinity of the middle at 30 μm or more is required for decomposing a band-shaped structure formed in production steps. A crystal particle diameter d1 of less than 30 μm prevents a band-shaped structure causing the generation of a ridging mark from being sufficiently decomposed and results in the generation of a ridging mark. The crystal particle diameter d1 is preferably set at 45 μm or more, and still more preferably at 60 μm or more. In contrast, a crystal particle diameter d1 of more than 80 μm results in the great deterioration of elongation and formability, and therefore, the crystal particle diameter d1 is set at 80 μm or less, and preferably at 70 μm or less.

In the conventional findings, an increase in the crystal particle diameter of a final sheet has been considered to lead to the deterioration of surface quality and formability, and may cause, for example, a problem relating to surface quality such as surface quality called surface roughening or orange peel. A decease in crystal particle diameter is known to be effective for suppressing the deterioration of surface quality and formability. In the present disclosure, therefore, it is necessary to control a crystal particle diameter in a certain plane in the entire sheet thickness to such a degree that surface roughening and orange peel do not occur. Specifically, the crystal particle diameter d2 in an L-ST plane in the entire sheet thickness is set at 60 μm or less. The crystal particle diameter d2 is preferably set at 50 μm or less. When the crystal particle diameter d2 is less than 10 μm, the problem of surface quality called an SS mark may be prone to occur. Therefore, the lower limit value thereof is preferably set at 10 μm.

In the present disclosure, the control of a crystal orientation is required simultaneously with the control of the crystal particle diameters of the aluminum alloy sheet described above. The control of the crystal orientation of a sheet surface layer particularly having a strong influence on hem bendability is important for improving the hem bendability. To this end, the cube orientation area rate C of the crystal orientation in the sheet surface is set at 10% or more. The cube orientation area rate C is more preferably set at 15% or more. It may be considered that the setting of the upper limit thereof at 60% is appropriate because molding may become difficult as a result of an increase in that anisotropy of a material.

2-2. Method for Measuring Crystal Particle Diameters and Crystal Orientation

A specific method for measuring crystal particle diameters and a crystal orientation will be described. First, with regard to the crystal particle diameter d1 in an L-LT plane in the region in the vicinity of the middle of the aluminum alloy sheet, the thickness is decreased to an optional L-LT plane in the range of the region in the vicinity of the middle by caustic etching, and mechanical polishing, buffing-polishing, and electrolytic polishing are then performed to form a measurement plane. Then, with regard to the crystal particle diameter d2 in an L-ST plane in the entire sheet thickness of the aluminum alloy sheet, the mechanical polishing, buffing-polishing, and electrolytic polishing of an optional L-ST surface of the aluminum alloy sheet are performed to form a measurement plane. In addition, with regard to the cube orientation area rate C of a crystal orientation in a sheet surface, the mechanical polishing, buffing-polishing, and electrolytic polishing of the aluminum alloy sheet surface are performed to form a measurement plane.

Specifically, the orientation data of a texture is acquired by measuring each of the above-described measurement planes by a backscattered electron diffraction measurement apparatus (SEM-EBSD) attached to a scanning electron microscope. Then, the crystal particle diameters d1 and d2 are obtained from the obtained orientation data by using EBSD analysis software (“OIM Analysis” manufactured by TSL). A circle equivalent diameter calculated by regarding a crystalline boundary line having a misorientation of 5° or more as a crystal grain boundary is regarded as a crystal particle diameter. The crystal orientation area rate C is measured from the orientation data obtained in a similar manner by using the EBSD analysis software. The cube orientation area rate C is calculated by regarding a crystal orientation of 15° or less from (001) <100> orientation as a cube orientation. It is preferable that a measurement region on each measurement plane is set at an area of 1000 μm×1000 μm or more in the case of the L-LT plane or at an area of 1000 μm×1000 μm or more (or the total sheet thickness) in the case of the L-ST plane, and a measurement step spacing is set at around 1/10 of the crystal particle diameter. It is preferable that each of d1, d2, and C is determined based on the arithmetic mean value of values obtained by measuring three or more spots.

In the present disclosure, surface roughening and the like are prevented by controlling the crystal particle diameter d2 in the L-ST plane of the entire sheet thickness while increasing the crystal particle diameter d1 to an appropriate size in the L-LT plane in the region in the vicinity of the middle in recrystallization in solution treatment to decompose the band-shaped structure in order to improve ridging resistance, and hem bendability is improved by further controlling the cube orientation area rate C of the crystal orientation in the sheet surface, as described above. According to the present disclosure, the aluminum alloy sheet excellent in surface quality such as ridging resistance or surface roughening resistance and in hem bendability can be obtained.

A sheet thickness in which a ridging mark can be prevented by controlling crystal particle diameters as in the case of the present disclosure is not particularly restricted but can be applied to a final rolled sheet having a predetermined sheet thickness required by a product. This is because a band-shaped structure having a strong influence on the generation of a ridging mark is formed in a region in the vicinity of a middle (a region in a range of ±(t/8) from the center (t/2) of a sheet thickness, regarded as a center, along a thickness direction) after hot rolling, and the rate of the region in the vicinity of the middle to the total sheet thickness t is not changed even when rolling proceeds, and the sheet thickness is decreased. In fact, it may be specifically described that a sheet thickness preferably commonly used for the present disclosure is 0.5 to 5.0 mm.

3. Method for Producing Aluminum Alloy Sheet According to Present Disclosure

A method for producing an aluminum alloy sheet according to the present disclosure will now be described. With regard to the material structure of a final sheet, it is necessary to perform hot rolling, cold rolling, and solution treatment in a process of producing the sheet under a particular condition in order to control crystal particle diameters and a crystal orientation, in the present disclosure.

For obtaining the crystal particle diameters d1 and d2 and cube orientation area rate C defined in the present disclosure, it is necessary to allow a recrystallization particle diameter in the solution treatment to be larger than a recrystallization particle diameter generated in a conventional method. To that end, with regard to the constituent composition of the aluminum alloy, it is effective to allow the amounts of Mn and Cr which are elements that suppress crystal grain growth to be smaller than usual. The setting of the amount of Mn+Cr in the range defined in the present disclosure results in a moderate increase in crystal particle diameter in the solution treatment and enables a particle diameter d1 of 30 μm to 80 μm to be obtained in a region in the vicinity of the middle of a sheet thickness.

However, a decrease in the amounts of Mn and Cr which are the elements that suppress crystal grain growth causes the possibility of coarsening crystal grains in the hot rolling before the solution treatment. Particularly when the hot rolling is performed at not less than a recrystallization temperature, a crystal particle diameter is excessively increased by recrystallization. In this regard, the present inventors have obtained findings that the refinement of a band-shaped structure formed in hot rolling is preferred for effectively decomposing a band-shaped structure by recrystallized grains in solution treatment, as described above. Thus, a hot-rolling temperature is restricted to suppress recrystallization in hot rolling, thereby preventing coarsening recrystallization in the hot rolling, in the present disclosure.

After the hot rolling, the cold rolling can be performed until achieving a final sheet thickness without performing intermediate annealing. A rolling texture can be sufficiently grown to obtain the cube orientation area rate C defined in the present disclosure because the intermediate annealing is not performed. It is necessary to appropriately control a cold rolling reduction in order to obtain the predetermined crystal particle diameters and cube orientation density, and a feature such as ridging resistance by the end temperature of the hot rolling.

The above-described method for producing an aluminum alloy sheet according to the present disclosure has technological significance in the restriction of the constituent composition of the aluminum alloy, the control of the hot-rolling temperature in the hot rolling, the omission of the intermediate annealing, and the control of the cold rolling reduction. Accordingly, the method can be carried out in steps of which the number is equal to or less than the number of steps currently commonly used in a method for producing an automobile body sheet material, that is, steps in order of casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, cold rolling, and solution treatment. This is also a favorable feature from the viewpoint of a reduction in the cost of an automobile body sheet material.

A typical and preferable method for producing an aluminum alloy sheet having the above d1, d2, and C defined in the present disclosure will be described below. In the method for producing the aluminum alloy sheet, a molten aluminum alloy is cast, optionally subjected to homogenization treatment, and then subjected to hot rolling, cold rolling, and solution treatment in the order mentioned above.

3-1 Casting Step

The aluminum alloy having the constituent composition described above is melted according to a usual method, and cast by a usual casting method such as a continuous casting method (CC casting method) or a semi-continuous casting method (DC casting method), which is selected as appropriate.

3-2. Homogenization Treatment Step

Homogenization treatment of an ingot obtained in the casting step may be performed as needed. As the homogenization treatment, heat treatment is preferably performed at a temperature of 480 to 590° C. for 0.5 to 24 hours.

3-3. Hot-Rolling Step

Hot rolling of the ingot subjected to the homogenization treatment or the cast ingot in the case of performing no homogenization treatment is performed. Any of the following treatment methods can be applied as needed in a process from the homogenization treatment step or the casting step to the start of the hot rolling.

In other words, in the case of performing the homogenization treatment, there is a method in which the ingot is cooled to ordinary temperature or a temperature around ordinary temperature in a cooling process after the homogenization treatment, then heated again to a temperature at which the hot rolling is started, and retained (preheated) for 24 h or less as needed, and the hot rolling is started at the temperature. In addition, there is a method in which the ingot is cooled to the temperature at which the hot rolling is started in the cooling process after the homogenization treatment, and retained (preheated) for 24 h or less as needed, and the hot rolling is started at the temperature. In any preheating, the lower limit of retention time is not particularly limited, and the hot rolling may be started immediately after reaching the predetermined temperature. A higher cooling rate after the homogenization treatment is preferred because of facilitating improvement in mechanical characteristics such as ASYA, ASEL, and BHYS required for an automobile body sheet material.

In the case of performing no homogenization treatment, there is a method in which the ingot is cooled to ordinary temperature or a temperature around ordinary temperature after the casting step, then heated again to the temperature at which the hot rolling is started, and retained for 24 h or less, and the hot rolling is started at the temperature. The lower limit of retention time is not particularly limited, and the hot rolling may be started immediately after reaching the predetermined temperature.

A common step of hot-rolling an aluminum alloy is adopted in the fundamental content of the hot rolling. In the present disclosure, however, it is essential to restrict hot-rolling conditions in order to suppress excessive coarsening of crystal grains in the hot-rolling step, as described above. A hot-rolling start temperature and a hot-rolling end temperature are defined for hot-rolling temperatures. In the present disclosure, the hot-rolling start temperature is set at 300 to 450° C., and the hot-rolling end temperature is set at 200 to 450° C.

The hot-rolling conditions are classified roughly into the following two conditions A and B. In the condition A, the hot-rolling start temperature is set at 300 to 450° C., and the hot-rolling end temperature is set at 200 to 350° C. A hot-rolling start temperature of less than 300° C. precludes rolling or results in the remarkable deterioration of productivity. In contrast, a hot-rolling start temperature of more than 450° C. is prone to result in recrystallization in the hot rolling, causes the formation of an excessively coarse recrystallized structure in the aluminum alloy including small amounts of Mn and Cr, used in the present disclosure, and may result in the generation of a strong ridging mark in a product sheet. In addition, a hot-rolling end temperature of less than 200° C. precludes rolling. In contrast, a hot-rolling end temperature of more than 350° C. may result in recrystallization, thereby forming a coarse recrystallized grain structure. As described above, it is preferable to set the hot-rolling start temperature at 300 to 450° C. and to set the hot-rolling end temperature at 200 to 350° C. Under the condition A, it is more preferable to set the hot-rolling start temperature at 300 to 420° C. and to set the hot-rolling end temperature at 200 to 300° C.

Under the condition B, the hot-rolling start temperature is set at 350 to 450° C., and the hot-rolling end temperature is set at more than 350° C. and 450° C. or less. In such a case, the hot-rolling end temperature of the condition B is in a temperature range in which the coarse recrystallized grain structure may be formed under the condition A, but is a condition which has been found to enable a band-shaped structure causing a ridging mark to be decomposed by appropriately controlling a subsequent cold rolling reduction even when a temperature exceeds 350° C. which is the condition of the hot-rolling end temperature described in the condition A.

The condition B has a feature that the hot-rolling start temperature is set at 350 to 450° C., and a lower limit value is set at a higher temperature than the lower limit value of the condition A. The feature enables a cooling temperature range from the homogenization treatment or the casting to the hot rolling to be narrowed, thereby saving energy. The upper limit value of the hot-rolling start temperature is set at 450° C. which is the same as that of the condition A. The reason thereof is also the same as that of the condition A and is because recrystallization is prone to occur in the hot rolling at more than 450° C. In contrast, the hot-rolling end temperature is set at more than 350° C. and 450° C. or less. This range is wider than the range defined in the condition A. In combination with the setting of the hot-rolling start temperature at 350° C. or more, it is easy to adjust the hot-rolling end temperature to a target temperature. The reason why the hot-rolling end temperature is set at 450° C. or less is because a hot-rolling end temperature of more than 450° C. causes recrystallized grains in the hot rolling to be excessively coarsened, thereby preventing a band-shaped structure from being sufficiently decomposed in subsequent steps. In the condition B, it is preferable to set the hot-rolling start temperature at 350 to 450° C. and to set the hot-rolling end temperature at more than 350° C. and 450° C. or less, as described above. In the condition B, it is more preferable to set the hot-rolling start temperature at 370 to 450° C. and to set the hot-rolling end temperature at more than 350° C. and 420° C. or less.

3-4. Cold-Rolling Step

Subsequently to the hot-rolling step, cold rolling of the hot-rolled sheet is performed to form a cold-rolled sheet having a final sheet thickness (product sheet thickness). The desired cold rolling reductions in the condition A and condition B of the hot rolling described above are different from each other. In the case of the condition A of the hot rolling, the rolling reduction in the cold-rolling step is set at 50.0% or more, and preferably at 66.0% or more. A rolling reduction of less than 50.0% prevents a rolling texture formed in the rolling from being sufficiently grown and results in an insufficient cube orientation area rate C formed in solution treatment. The upper limit value of the rolling reduction, which depends on a facility used, is set at 90% from the viewpoint of productivity. A ridging mark can be more reliably prevented in a material having a hot-rolling end temperature of 300° C. to 350° C. in the condition A by setting a cold rolling reduction at 76.5% or more. The reason thereof is the same as that of the condition B of the hot rolling described below.

In the case of the condition B of the hot rolling, the hot-rolling end temperature is high, and strain energy accumulated at the time of the completion of the hot rolling by recovering is less than that of the hot-rolling condition A. A technique in which a cold rolling reduction is higher than that in the condition A is adopted in order to grow a rolling texture contributing to the formation of a cube orientation to a level equivalent to that of the condition A. The higher cold rolling reduction promotes recrystallization around second phase particles existing in a material structure (particle-stimulated nucleation), thereby resulting in the more effective decomposition of a band-shaped structure. The hot-rolling end temperature in the condition B is higher than that in the condition A, and coarse recrystallized grains are easily formed in the hot-rolling. However, it is expected that ridging resistance is secured by enhancing the power of the decomposition of a band-shaped structure by enhancing the cold rolling reduction. In the case of the condition B, the rolling reduction in the cold-rolling step is preferably set at 76.5% or more, and more preferably at 80.0% or more. A rolling reduction of 76.5% or more results in the sufficient growth of a rolling texture formed in the rolling, in a sufficient cube orientation area rate C formed in solution treatment, and in sufficient ridging resistance. The preferred upper limit value of the rolling reduction in the condition B, which depends on a facility used, is set at 90% from the viewpoint of productivity.

3-5. Solution Treatment Step

Subsequently to the cold rolling, solution treatment of the rolled sheet is performed. A material achieving temperature in the solution treatment is 480 to 590° C., and preferably 500 to 590° C. A material achieving temperature of less than 480° C. may prevent recrystallization and may result in insufficient solution, thereby preventing strength meeting an objective from being obtained. In contrast, a material achieving temperatures of more than 590° C. may cause the sheet to be melted, thereby precluding stable production. Retention time in the solution treatment is not particularly limited, and is preferably 0 second to 5 minutes, and more preferably 0 second to 1 minute from the viewpoint of productivity. In such a case, “0 second” means that cooling is performed immediately after reaching the material achieving temperature. In the cooling after the solution treatment, a cooling rate in a temperature range from a retention temperature to 150° C. is preferably set at 100° C./min or more, whereby sufficient formability and bake hardenability can be obtained. The cooling rate is more preferably set at 300° C./min or more. The upper limit value of the cooling rate, which depends on a cooling apparatus and a cooling method, is set at 10000° C./min from the viewpoint of productivity and operability in the present disclosure.

3-6. Other Step

In the present disclosure, it is preferable to perform preliminary aging treatment immediately after the solution treatment in order to obtain favorable bake hardenability. The conditions of the preliminary aging treatment are preferably a temperature of 50 to 150° C. and a retention time of 1 to 100 hours. However, the preliminary aging treatment has no essential influence on crystal particle diameters and a crystal orientation. Accordingly, it is not necessary to perform the preliminary aging treatment in the present disclosure.

Examples

Examples of the present disclosure, together with Comparative Examples, will be described below.

In the present disclosure, the constitution of the disclosure, achieving improvement in ridging resistance and hem bendability, which is a principal challenge of the present disclosure, includes the crystal particle diameters d1 and d2 and cube orientation area rate C described above. Thus, the above-described constitution of the disclosure is controlled by defining the constituent composition of the alloy and the conditions of the production steps in the specific ranges in the present Examples. A surface roughening property is also suppressed by d1 and d2, and therefore, the surface roughening property was also evaluated in the present Examples. In addition to such principal challenges, strength as a mechanical characteristic was also further evaluated in the present Examples. This is because strength is a feature inherently required by an automotive body sheet and the like although examples of the challenges of the present disclosure do not include strength.

The following Examples are intended to explain the effects of the present disclosure, and processes and conditions described in Examples do not restrict the technical scope of the present disclosure.

[Production of Aluminum Alloy Sheet Material]

An aluminum alloy having a constituent composition denoted by each of alloy reference characters A to S in Table 1 was melted according to a usual method, and cast into a slab by a DC casting method. Then, each obtained slab was subjected to homogenization treatment, and spontaneously cooled to a temperature around room temperature. Hot rolling, cold rolling, and solution treatment of the slab subjected to the homogenization treatment were performed to form a sheet material sample. In addition, a sheet material sample was also produced by performing homogenization treatment, then performing cooling to a hot-rolling start temperature, and performing hot rolling, cold rolling, and solution treatment on an as-is basis. Such production steps will be described below while setting forth treatment conditions in Table 2.

TABLE 1 Alloy reference Alloy constituent composition (unit: mass %) character Si Mg Cu Fe Mn Cr Zn Zr Ti Al Mn + Cr Application A 1.00 0.60 — 0.14 0.080 — — — 0.100 Balance 0.080 Disclosure B 1.40 0.40 0.70 0.90 0.020 0.020 — 0.10 0.020 Balance 0.040 Example C 1.90 0.40 — 0.21 0.040 — 1.35 — 0.200 Balance 0.040 D 0.40 1.40 1.40 0.21 0.010 0.070 0.06 — 0.020 Balance 0.080 E 1.30 0.70 — 0.10 — 0.020 — — 0.020 Balance 0.020 F 1.20 0.30 0.70 0.20 — 0.080 2.31 — 0.260 Balance 0.080 G 1.00 0.60 — 0.20 0.050 0.050 — — 0.100 Balance 0.100 Comparative H 1.00 0.60 — 0.20 0.100 — — — 0.100 Balance 0.100 Example I 1.00 0.60 — 0.20 — 0.100 — — 0.100 Balance 0.100 J 1.00 0.60 — 0.20 0.100 0.100 — — 0.100 Balance 0.200 K 1.00 0.60 — 0.20 0.003 — — — 0.100 Balance 0.003 L 1.00 0.60 — 0.20 — 0.003 — — 0.100 Balance 0.003 M 0.15 0.61 — 0.18 0.020 0.020 — — 0.010 Balance 0.040 N 1.00 0.10 0.71 0.19 0.020 0.020 — — 0.020 Balance 0.040 O 2.50 0.60 — 0.20 0.020 0.020 — — 0.010 Balance 0.040 P 1.00 2.00 0.70 0.14 0.020 0.020 — — 0.020 Balance 0.040 Q 1.01 0.60 1.80 1.12 0.020 0.020 3.02 — 0.110 Balance 0.040 R 1.00 0.61 — 0.21 0.020 0.020 — 0.51 0.100 Balance 0.040 S 1.00 0.59 — 0.21 0.020 0.020 — — 0.430 Balance 0.040 Each designation “—” in the table represents that a corresponding constituent is not added.

TABLE 2 Hot- Sheet Solution Preheating rolling thick- treatment condition start Hot-rolling ness Cold Presence or Achieving Reten- Production Alloy Cooling after before hot temper- end after hot rolling absence of temper- tion process reference homogenization rolling ature temperature rolling reduction intermediate ature time No. character treatment (° C.) (° C.) (° C.) (mm) (%) annealing (° C.) (sec) Category 1 A Spontaneous 350 350 250 6.0 83.3 Absent 580 0 Disclosure cooling to room Example temperature 2 A Spontaneous 450 450 350 3.0 66.7 Absent 580 0 Disclosure cooling to room Example temperature 3 A Spontaneous 350 350 360 4.3 76.7 Absent 580 0 Disclosure cooling to room Example temperature 4 A Spontaneous 400 400 370 4.5 77.8 Absent 580 0 Disclosure cooling to room Example temperature 5 A Spontaneous 400 400 380 5.0 80.0 Absent 580 0 Disclosure cooling to room Example temperature 6 A Spontaneous 450 450 400 4.5 77.8 Absent 580 0 Disclosure cooling to room Example temperature 7 A Spontaneous 450 450 450 6.0 83.3 Absent 580 0 Disclosure cooling to room Example temperature 8 A Cooling to 400 400 350 3.0 66.7 Absent 580 0 Disclosure hot-rolling start Example temperature 9 A Cooling to 400 400 380 5.0 80.0 Absent 580 0 Disclosure hot-rolling start Example temperature 10 A Cooling to 450 450 400 4.5 77.8 Absent 580 0 Disclosure hot-rolling start Example temperature 11 B Spontaneous 400 400 300 2.0 50.0 Absent 580 10 Disclosure cooling to room Example temperature 12 B Cooling to 400 400 300 2.0 50.0 Absent 580 10 Disclosure hot-rolling start Example temperature 13 C Spontaneous 300 300 200 6.0 83.3 Absent 480 300 Disclosure cooling to room Example temperature 14 C Spontaneous 300 300 200 4.0 75.0 Absent 520 0 Disclosure cooling to room Example temperature 15 D Spontaneous 350 350 250 8.0 87.5 Absent 520 60 Disclosure cooling to room Example temperature 16 E Spontaneous 350 350 250 4.0 75.0 Absent 520 0 Disclosure cooling to room Example temperature 17 E Spontaneous 400 400 380 6.0 83.3 Absent 520 0 Disclosure cooling to room Example temperature 18 E Cooling to 400 400 380 6.0 83.3 Absent 520 0 Disclosure hot-rolling start Example temperature 19 F Spontaneous 350 350 250 4.0 75.0 Absent 480 0 Disclosure cooling to room Example temperature 20 A Spontaneous 500 500 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 21 A Spontaneous 480 480 380 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 22 A Spontaneous 500 500 470 4.5 77.8 Absent 550 10 Comparative cooling to room Example temperature 23 A Spontaneous 400 400 250 1.5 33.3 Absent 550 10 Comparative cooling to room Example temperature 24 A Spontaneous 400 400 250 4.0 75.0 Absent 450 10 Comparative cooling to room Example temperature 25 A Spontaneous 400 400 250 4.0 75.0 Present 550 10 Comparative cooling to room Example temperature 26 G Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 27 H Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 28 I Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 29 J Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 30 K Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 31 L Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 32 M Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 33 N Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 34 O Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 35 P Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 36 Q Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 37 R Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature 38 S Spontaneous 400 400 250 4.0 75.0 Absent 550 10 Comparative cooling to room Example temperature

Each cast slab was subjected to homogenization treatment under conditions of 530° C. and 8 hours, and then to cooling after the homogenization treatment, preheating before hot rolling, and the hot rolling in turn under various conditions set forth in Table 2. In a case in which the cooling after the homogenization treatment is expressed as “spontaneous cooling to room temperature”, cooling to room temperature was temporarily performed after the homogenization treatment, reheating was performed from the room temperature to a hot-rolling start temperature, and preheating in which retainment was performed at the temperature for 2 hours was performed. In a case in which the cooling after the homogenization treatment is expressed as “cooling to hot start temperature”, cooling to the hot-rolling start temperature was performed after the homogenization treatment, and preheating in which retainment was performed at the temperature for 10 minutes was performed. In addition, after both types of the preheating, hot rolling was performed at a start temperature and an end temperature set forth in Table 2. A sheet thickness after the hot rolling is set forth in Table 2.

Then, cold rolling of the hot-rolled sheet was performed at a cold rolling reduction set forth in Table 2, to obtain a cold-rolled sheet having a final sheet thickness of 1.0 mm. In production process No. 25, a hot-rolled sheet of 4 mm was subjected to cold rolling to 2.0 mm, then subjected to intermediate annealing at 500° C. for a retention time of 10 seconds by using a continuous annealing furnace, and then subjected to cold rolling again to obtain a cold-rolled sheet having a final sheet thickness of 1.0 mm.

Solution treatment was performed under the conditions set forth in Table 2. A continuous annealing furnace was used for the solution treatment. Then, cooling was performed to a temperature around room temperature at a cooling rate of 600 to 1000° C./min, immediately followed by performing preliminary aging treatment at 80° C. for 5 hours. The preliminary aging treatment has an influence on mechanical properties but has no influence on crystal particle diameters and a crystal orientation.

[Measurement of Crystal Particle Diameters and Crystal Orientation]

The crystal particle diameters d1 and d2 and cube orientation area rate C of each sheet material sample with a final sheet thickness of 1 mm, obtained as described above, was measured by the method described above. In the present Examples, a sample was collected from a region within 50 mm from the center of the sheet material in a width direction, and the crystal particle diameters and cube orientation area rate C of the sample were measured. In the measurement of the particle diameter d1, the particle diameter d1 of an L-LT plane at a position having a thickness of ±0.05 mm from the center (t/2) of the sheet thickness was measured. In the measurement of the particle diameter d2, the particle diameter d2 of the L-ST plane of the collected sample was measured. The cube orientation area rate C of an L-LT plane obtained by polishing a surface of the collected sample to a thickness of about 60 to 100 μm was measured.

[Test for Evaluating Ridging Resistance and Surface Roughening Property]

The ridging resistance of each sheet material sample was evaluated using a conventionally performed simple evaluation technique. Specifically, JIS No. 5 test pieces were collected along a direction at 90° with respect to a rolling direction. The test pieces were subjected to 5% and 15% stretches, respectively. Assuming that a stripe pattern (stripe-shaped recessed and projected pattern) generated on a surface along the rolling direction was regarded as a ridging mark, the presence or absence of generation of the stripe pattern was determined by visual observation. The 5% stretch is equivalent to a strain amount on the assumption of usual press molding, while the 15% stretch is equivalent to a strain amount on the assumption of particularly severe molding. “Good” represents no stripe pattern, and “Poor” represents the state of a clear stripe pattern. Similarly, the presence or absence of surface roughening was also determined. “Good” represents no occurrence of surface roughening, and “Poor” represents that surface roughening of which the degree was problematic for surface quality occurred. The results are set forth in Table 3.

[Test for Evaluating Hem Bendability]

A hemming test for evaluating the hem bendability of each sheet material sample was conducted. In the hemming test, JIS No. 5 test pieces in a direction at 90° with respect to a rolling direction were collected 90 days after a day on which the solution treatment was performed. The hemming test was conducted based on JISH7701. A prestrain was set at 8%, a punch tip radius in pre-hemming was set at 0.5 mm, and the thickness of an intermediate sheet in this hemming was set at 1.0 mm. After the hemming test, the surface of the outer periphery was observed, 0 to 2 points described in JISH7701 were evaluated as acceptable, “Good”, and 3 to 4 points were evaluated as unacceptable, “Poor”.

[Strength Evaluation Test]

Finally, strength was also evaluated as a mechanical characteristic. A JIS No. 5 test piece was cut from each sheet material sample produced as described above in a direction parallel to the rolling direction 7 days after the day on which the solution treatment was performed, and the 0.2% yield strength (ASYS) and elongation (ASEL) of the test piece were evaluated by a tensile test. A 0.2% yield strength value (BHYS) obtained by stretching the test piece by 2% and then performing heat treatment of the test piece in an oil bath at 170° C. for 20 minutes as heat treatment corresponding to coating baking treatment was also measured. An ASYS of 90 MPa or more, an ASEL of 25% or more, and a BHYS of 160 MPa or more were evaluated as acceptable, “Good”, and the other ranges were evaluated as unacceptable, “Poor” on the basis of criteria required for an automobile body sheet material as criteria for determining formability and strength. The results are set forth in Table 3.

The measurement results of the crystal particle diameters and crystal orientation of each aluminum alloy sheet material sample produced by the present Examples, and the evaluation results relating to the ridging resistance, surface roughening property, hem bendability, and strength of the aluminum alloy sheet material sample are set forth in Table 3.

TABLE 3 Crystal particle Cube Ridging diameter orientation area resistance Surface Production d1 d2 rate C 5% 15% roughening Hemming- ASYA ASEL BHYS process No. (μm) (μm) (%) stretch stretch resistance bending (MPa) (%) (MPa) category 1 32 29 38 Good Good Good Good 106 Good 30 Good 225 Good Disclosure Example 2 38 34 28 Good Good Good Good 100 Good 28 Good 220 Good Disclosure Example 3 34 32 30 Good Good Good Good 102 Good 29 Good 222 Good Disclosure Example 4 36 33 34 Good Good Good Good 105 Good 29 Good 218 Good Disclosure Example 5 33 31 29 Good Good Good Good 104 Good 28 Good 220 Good Disclosure Example 6 31 30 28 Good Good Good Good 98 Good 27 Good 220 Good Disclosure Example 7 37 33 34 Good Good Good Good 100 Good 28 Good 216 Good Disclosure Example 8 36 34 38 Good Good Good Good 108 Good 28 Good 228 Good Disclosure Example 9 38 31 37 Good Good Good Good 112 Good 29 Good 232 Good Disclosure Example 10 35 30 28 Good Good Good Good 113 Good 28 Good 233 Good Disclosure Example 11 45 36 18 Good Good Good Good 103 Good 30 Good 218 Good Disclosure Example 12 45 35 22 Good Good Good Good 105 Good 29 Good 225 Good Disclosure Example 13 45 43 35 Good Good Good Good 121 Good 30 Good 228 Good Disclosure Example 14 50 48 32 Good Good Good Good 117 Good 29 Good 228 Good Disclosure Example 15 38 34 38 Good Good Good Good 123 Good 31 Good 226 Good Disclosure Example 16 78 59 26 Good Good Good Good 121 Good 29 Good 229 Good Disclosure Example 17 65 55 29 Good Good Good Good 118 Good 29 Good 226 Good Disclosure Example 18 70 54 34 Good Good Good Good 124 Good 29 Good 233 Good Disclosure Example 19 36 27 28 Good Good Good Good 112 Good 27 Good 212 Good Disclosure Example 20 38 34 8 Poor Poor Good Poor 115 Good 29 Good 215 Good Comparative Example 21 37 29 4 Poor Poor Good Poor 117 Good 28 Good 217 Good Comparative Example 22 50 41 16 Poor Poor Good Good 115 Good 27 Good 214 Good Comparative Example 23 38 35 5 Good Good Good Poor 115 Good 27 Good 215 Good Comparative Example 24 40 37 26 Good Good Good Good 75 Poor 29 Good 130 Poor Comparative Example 25 39 35 3 Good Good Good Poor 93 Good 29 Good 175 Good Comparative Example 26 25 26 21 Good Poor Good Good 112 Good 28 Good 220 Good Comparative Example 27 26 24 20 Good Poor Good Good 110 Good 29 Good 218 Good Comparative Example 28 25 28 24 Good Poor Good Good 113 Good 28 Good 215 Good Comparative Example 29 20 23 23 Good Poor Good Good 112 Good 30 Good 215 Good Comparative Example 30 90 80 23 Good Good Poor Good 103 Good 26 Good 207 Good Comparative Example 31 90 80 24 Good Good Poor Good 105 Good 27 Good 206 Good Comparative Example 32 45 39 24 Good Good Good Good 74 Poor 29 Good 135 Poor Comparative Example 33 48 41 23 Good Good Good Good 76 Poor 28 Good 134 Poor Comparative Example 34 46 45 24 Good Good Good Good 130 Good 24 Poor 240 Good Comparative Example 35 48 40 28 Good Good Good Good 125 Poor 24 Poor 250 Good Comparative Example 36 42 39 25 Good Good Good Good 124 Poor 23 Poor 248 Good Comparative Example 37 49 36 22 Good Good Good Good 132 Good 23 Poor 254 Good Comparative Example 38 49 39 26 Good Good Good Good 128 Good 21 Poor 138 Good Comparative Example

In each of the examples of production process Nos. 1 to 19 in Table 3, the constituent composition of the alloy is in the range defined in this disclosure, and crystal particle diameters d1 and d2 and a cube orientation area rate C after the solution treatment are in the ranges defined in the present disclosure. In each of these examples, no ridging mark was generated, and no surface roughening occurred in the 5% and 15% stretches, and it was confirmed that there were no problems in both stretches. All of ASYA, ASEL, and BHYS as the mechanical properties required for an automobile body sheet material sufficiently satisfied required performances.

In contrast, in each of production process Nos. 20 and 21 in Table 3, the hot-rolling start temperature was high, it was impossible to sufficiently decompose coarse recrystallization proceeding in the hot rolling by recrystallization in the subsequent cold rolling and solution treatment, and ridging resistance was deteriorated. Since a rolling texture was not sufficiently grown even in the cold rolling, a cube orientation area rate C was less than 10%, and hem bendability was poor.

In production process No. 22 in Table 3, both of a hot-rolling start temperature and a hot-rolling end temperature were too high. Therefore, even when the cold rolling was performed at a high rolling reduction, it was impossible to sufficiently decompose coarse recrystallization proceeding in the hot rolling by recrystallization in the subsequent cold rolling and solution treatment, and ridging resistance was deteriorated.

In production process No. 23 in Table 3, a cold rolling reduction was low in the process of producing the sheet material sample, and therefore, a cube orientation area rate C was outside the range of the area rate (10%) defined in the present disclosure. As a result, hem bendability was poor. In the sheet material sample, the temperatures of the hot rolling (start temperature and end temperature) were appropriately set, and crystal particle diameters d1 and d2 were within the ranges defined in the present disclosure. Therefore, no ridging mark was generated, and no surface roughening occurred even in the 15% stretch simulating severe press molding.

In production process No. 24 in Table 3, a solution treatment temperature was low in the process of producing the sheet material sample, and therefore, mechanical properties required for an automobile body sheet material were not satisfied. The constituent composition of the alloy, temperatures at which the hot rolling was started and ended, and a cold rolling reduction were appropriate, and therefore, crystal particle diameters d1 and d2 and a cube orientation area rate C were within the ranges defined in the present disclosure. Therefore, no ridging mark was generated, and no surface roughening occurred even in the 15% stretch, and sufficient hem bendability was able to be obtained.

In production process No. 25 in Table 3, a cube orientation area rate C was lower than the area rate defined in the present disclosure because the intermediate annealing was performed between the cold-rolling steps in the process of producing the sheet material sample. As a result, hem bendability was poor.

Each of production process Nos. 26 to 31 in Table 3 is an example in which the constituent composition of the alloy is outside the range defined in the present disclosure. In each of the production process Nos. 26 to 29, a crystal particle diameter d1 after the solution treatment was smaller than the range defined in the present disclosure because the amount of Mn+Cr was more than 0.080%. In each of these examples, a ridging mark was generated in the 15% stretch simulating severe press molding although no ridging mark was generated in the 5% stretch because temperatures at which the hot rolling was started and ended are in the ranges defined in the present disclosure. In each of production process Nos. 30 and 31, the amount of Mn+Cr was less than 0.005%, and therefore, both of crystal particle diameters d1 and d2 after the solution treatment were larger than the ranges defined in the present disclosure, whereby surface roughening occurred.

Each of production process Nos. 32 to 35 in Table 3 is an alloy to which either of Si and Mg which were essential additional elements was added in an amount that was less or more than the addition range defined in the present disclosure. Operation conditions in production conditions were allowed to be the conditions of the present disclosure, whereby each of these sheet material samples included crystal particle diameters d1 and d2 and a cube orientation area rate C. As a result, ridging mark resistance, hem bendability, and surface roughening resistance were acceptable. However, at least either elongation (ASEL) or 0.2% yield strength (ASYA, BHYS) did not satisfy the criterion of a mechanical property for an automobile body sheet material. Since Si and Mg are essential additional elements, it is found that the amounts of added Si and Mg should be allowed to be appropriate amounts also set in consideration of mechanical properties. However, the subject matter of the present disclosure is improvement in ridging resistance and hem bendability by allowing production conditions to be appropriate while regulating the amounts of added Mn and Cr, and in this respect, there is no problem.

Each example of production process Nos. 36 to 38 in Table 3 is an example in which the contents of essential elements (Si, Mg, Mn, and Cr) are within the appropriate ranges, and the production conditions thereof satisfy the ranges of the present disclosure, but at least any of Cu, Fe, Zn, Zr, and Ti which are selective elements is excessively added. Since the contents of Si, Mg, Mn, and Cr were within appropriate ranges, crystal particle diameters d1 and d2 and a cube orientation area rate C were satisfied, and ridging mark resistance, hem bendability, and surface roughening resistance were acceptable. However, since the selective elements are excessively included, at least any of ASYA, ASEL, and BHYS is “Poor”, and a mechanical property for an automobile body sheet material is not included. Thus, in the case of adding these optional additional elements, an alloy composition set in consideration of the preferred ranges of the elements should be applied.

INDUSTRIAL APPLICABILITY

The aluminum alloy sheet according to the present disclosure can allow the generation of a ridging mark to be reliably suppressed even under a severe molding condition, and has excellent hem bendability in view of formability. The method for producing an aluminum alloy sheet according to the present disclosure enables reliable and stable production at low cost on a mass production scale. The present disclosure can be utilized in molded components such as the panels and chassis of electronic and electrical instruments and the like, as well as in automotive applications such as automotive body sheets applied to the body panels of automobiles. 

1. An aluminum alloy sheet with excellent ridging resistance and excellent hem bendability, the aluminum alloy sheet comprising: an aluminum alloy including 0.20 to 1.50 mass % Mg, 0.30 to 2.00 mass % Si, and one or two of 0.005 to 0.080 mass % Mn and 0.005 to 0.080 mass % Cr, satisfying 0.005 mass %≤Mn+Cr≤0.080 mass %, and comprising balance Al and inevitable impurities, wherein the aluminum alloy sheet has a sheet thickness t, a position of a middle (t/2) of the sheet thickness is regarded as a center, and a crystal particle diameter d1 of an L-LT plane in a sheet thickness in a range of ±(t/8) from the center is 30 to 80 μm, a crystal particle diameter d2 of an L-ST plane in the entire sheet thickness is 60 μm or less, and a cube orientation area rate C of a crystal orientation on a sheet surface is 10% or more.
 2. The aluminum alloy sheet with excellent ridging resistance and excellent hem bendability according to claim 1, wherein the aluminum alloy further including one or more of 0.01 to 0.40 mass % Zr, 0.03 to 1.00 mass % Fe, 0.005 to 0.300 mass % Ti, and 0.03 to 2.50 mass % Zn, and includes Cu restricted to 1.50 mass % or less.
 3. A method for producing the aluminum alloy sheet with excellent ridging resistance and excellent hem bendability according to claim 1, the method comprising: a casting step of casting the aluminum alloy; a hot-rolling step of hot-rolling an ingot to form a hot-rolled sheet; a cold-rolling step of cold-rolling the hot-rolled sheet without subjected the hot-rolled sheet to intermediate annealing, to form a cold-rolled sheet; and a solution treatment step of performing solution treatment of the cold-rolled sheet, wherein in the hot-rolling step, a hot-rolling start temperature is set at 300 to 450° C., and a hot-rolling end temperature is set at 200 to 450° C.; in the cold-rolling step, the cold-rolled sheet having a final sheet thickness is formed at a rolling reduction of 50.0% or more; and in the solution treatment step, the solution treatment of the cold-rolled sheet is performed at a temperature of 480 to 590° C.
 4. The method for producing an aluminum alloy sheet with excellent ridging resistance and excellent hem bendability according to claim 3, wherein the hot-rolling start temperature is set at 300 to 450° C., and the hot-rolling end temperature is set at 200 to 350° C.
 5. The method for producing an aluminum alloy sheet with excellent ridging resistance and excellent hem bendability according to claim 3, wherein the hot-rolling start temperature is set at 350 to 450° C., and the hot-rolling end temperature is set at more than 350° C. and 450° C. or less.
 6. The method for producing an aluminum alloy sheet with excellent ridging resistance and excellent hem bendability according to claim 3, the method further comprising a homogenization treatment step of performing homogenization treatment of the ingot at a temperature of 480 to 590° C. for 0.5 to 24 hours between the casting step and the hot-rolling step. 